Technical Field
The present disclosure relates to a method for the selective production of para-isomers of dialkylbenzenes using a ZSM-5-silylated catalyst, a method for making the ZSM-5-silylated catalyst, and a method for the selective production of para-isomers where the selectivity of the para-dialkylbenzene isomers is at least 95 wt. % para-selectivity.
Description of the Related Art
The “background” description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description which may not otherwise qualify as prior art at the time of filing, are neither expressly or impliedly admitted as prior art against the present invention.
Xylenes encompass three isomers, namely ortho-, meta- and para-, of dimethylbenzene. para-Xylene is the principal precursor to terephthalic acid and dimethyl terephthalate, both monomers used in the production of polyethylene terephthalate plastic bottles and polyester clothing. 98% of para-xylene production, and half of all xylenes, is consumed in this way. The thermodynamic equilibrium yields only about a quarter of para-xylene, whereas its demand is much higher.
Ethyltoluenes also have three isomers namely ortho-, meta- and para-. The para isomer is used for the production of an important monomer, para-methylstyrene, by dehydrogenation. Poly(para-methylstyrene) has advantage over polystyrene due to its low density. It also has high glass transition temperature and flash point compared to polystyrene, therefore, it can have more potential application in the area of flame retardancy or ignition resistance. Packages made from poly(para-methylstyrene) are preferred for food packages which are subjected to high temperatures.
Diethylbenzenes have three isomers namely ortho-, meta- and para-. The para isomer is industrially more important than the other two isomers. It is a high-value chemical having immense industrial importance by virtue of its utility as a desorbent in the selective recovery of para-xylene from mixed xylene stream, by well known “Parex” adsorption process innovated by UOP, A Honeywell Company (USA).
Dialkylbenzenes have been traditionally synthesized using alkylation catalysts like AlCl3, HF, BF3 etc. However, the conventional catalyst is not selective to the para-isomer. Isomers of ortho-, meta- and para-dialkylbenzene can result in a thermodynamic equilibrium concentration. These isomers have very close boiling points to each other and the relative volatility is nearly one. Separation is difficult and is quite expensive. Moreover, due to strong acidity, disposal of catalyst causes serious environmental pollution apart from corrosion of equipment during operation of the process. Another approach for producing para-xylene (p-X), para-ethyltoluene (p-ET), and para-diethyl benzene (p-DEB) is through adsorptive separation of para-isomer from mixed diethyl benzene isomers, which are produced during ethylbenzene/styrene manufacture.
Solid acid catalysts, particularly zeolites have replaced the earlier AlCl3, HF, BF3 type catalysts. ZSM-5 is a type of zeolite catalyst which is used as solid acid catalyst having unique feature of shape selectivity. The pores of this sort of zeolites have a uniform aperture. Therefore, hydrocarbons smaller than the pore dimensions are adsorbed and larger hydrocarbons are rejected. ZSM-5 is frequently referred to as a “molecular sieve”. The ZSM-5 zeolite catalyst is characterized by its selectivity, being able to satisfy the needs for high selectivity to products of different molecules, but it still falls short of expectation in respect of isomers of same kind of product.
Various techniques to enhance shape selectivity of medium pore aluminosilicates have been reported. U.S. Pat. Nos. 4,086,287; 4,094,921 and 4,117,024 describe catalytic processes for selective ethylation of monoalkylbenzene (toluene, ethylbenzene) to produce p-X, p-ET, and p-DEB, using ZSM-5 catalyst modified with oxides of phosphorus, antimony, boron, magnesium and/or steaming and coking. U.S. Pat. Nos. 4,117,026 and 4,128,592 describe catalytic processes for the selective production of para dialkyl substituted benzenes using aluminosilicate zeolite modified with oxides which are difficult to reduce and further modified by coking. Catalysts and processes for selective production of para-dialkyl substituted benzenes have also been described in U.S. Pat. Nos. 4,379,761 and 4,465,886.
Enhancement of para-selectivity, (the fraction of para-isomer in a mixture of dialkyl aromatics), by treatment with organosilicon compound has been reported. It is carried out by contacting the zeolite with organosilicon compound, separation/removal of solvent (if used), and calcination of zeolite to deposit silica or polymeric silica as a layer on the zeolite. The efficiency of silica deposition in order to enhance the selectivity of the zeolite depends on the nature or the kind or the type or the molecular structure of the organosilicon compound employed. The efficiency of silica deposition also depends on the temperature of silylation, the solvents or the carrier for the organosilicon compound, the method or procedure adopted for the selectivation.
Silylation can be carried out in vapor phase or liquid phase. The zeolite is impregnated with an organosilicon compound dissolved or dispersed in a carrier or solvent followed by calcination of such treated zeolite in an oxygen containing atmosphere under conditions sufficient to remove organic material therefrom and deposit siliceous material on the zeolite. Such silylation may result in deposition of at least 1% by weight of siliceous material on the catalyst or zeolite.
Examples of various patents, which teach ex-situ selectivation of zeolites to enhance para-selectivity are U.S. Pat. No. 3,698,157, U.S. Pat. No. 4,002,697, U.S. Pat. Nos. 4,127,616 and 4,402,867. Silica modified catalysts employed for the purpose were based on zeolites like ZSM-5, ZSM-11 or ZSM-21, having surface deactivated by reaction with compounds of nitrogen or silicon, i.e. phenyl carbazole or dimethyldichlorosilane, (which are sufficiently large as to be unable to penetrate pores of crystalline aluminosilicate) followed by calcination. When ex-situ silylation process is repeated more than once, the procedure is referred to as ‘multiple silylation’ in which the zeolite is calcined after each impregnation of organosilicon compound.
While the above mentioned art is of interest, there is no suggestion of simultaneously enhancing the para-selectivity of dialkylbenzenes by its utilizating in short contact time fluidized-bed reactor. There is also no suggestion of multiple silylation of ZSM-5-based catalyst for ethylbenzene alkylation in the presence of methanol. Therefore, it would be a significant advance and improvement in the art through such a practical method using such modified catalyst.